The present invention relates to systems and methods for evaluating ocular and potentially systemic health based on the pointing direction of cone photoreceptors.
The Stiles-Crawford Effect of the First Kind (SCE-I) describes how rays of light entering near the center of the eye's pupil appear brighter to an observer than rays incident near the periphery. FIG. 1A illustrates this effect by showing a first centered ray of light 101 and a second off-center ray of light 103 entering the pupil 105 of the eye 107 and reaching photoreceptor cones 109 at the retina 111. The graph of FIG. 1B shows perceived intensity of the light (y-axis) as a function of displacement from center of the pupil (x-axis). As shown in FIG. 1B, the perceived intensity of the first light ray 101 is much higher (point 113) than the perceived intensity of the second light ray 103 (point 115 in the graph). This effect is due to the waveguide properties of the cone photoreceptors, and it has been demonstrated that various eye conditions such as amblyopia, retinal detachment and retinal degeneration have an altered SCE-I function.
Photoreceptor tilt can be measured based on the SCE-I. Prior measurement of the photoreceptor tilt has typically been performed psychophysically as a subjective comparison of the perceived intensity of a flickering stimulus relative to a static background. The stimulus would be moved across the subject's pupil to measure the luminance efficiency as a function of pupil eccentricity. This subjective method required lengthy experimental sessions and a high degree of concentration and cooperation from test subjects, making it impractical to be a routine clinical measurement technique.
Other, more time efficient techniques analyze the intensity distribution of reflected light at the pupil plane. The distribution of light reflected back from the retina, illuminated from a small source, is measured in the plane of the eye's pupil. The distribution of the reflected light returning from the retina presents a peaked profile. These pupil plane reflectometric techniques similarly provide an estimate of the pupil location towards which the photoreceptors are pointing; however the directionality is greater (higher rho values) than that measured psychophysically. Both of these approaches measure the properties of an ensemble of cones and are incapable of measuring the directionality of individual cones.